1. Field of the invention
This invention relates generally to overhead projectors for projecting images above or behind an operator, and more particularly to an overhead projector of the under mirror type in which a reflecting mirror is disposed on a lower portion of a projection head assembly including a projection lens and the mirror.
2. Description of the prior art
Overhead projectors are roughly classified into an over mirror type and an under mirror type. Overhead projectors of the over mirror type comprise a housing having a stage formed on the top thereof and a projection head assembly including a projection lens and a reflecting mirror. The projection head is supported on a column or mast so as to be disposed over the stage. Light from a light source is caused to pass through a transparency bearing an image to be projected and placed on the stage and then the projection lens. The light is then reflected on the mirror so that the image is focused on a screen. A projection angle is readily adjusted by adjustment of an angle of the mirror in the overhead projectors of the over mirror type. However, the mirror protrudes over the projection head assembly and this reduces an easiness to use the overhead projectors.
On the other hand, in overhead projectors of the under mirror type, light from a light source is caused to pass through a transparency and then to reflect on a reflecting mirror. The light is subsequently caused to pass through a projection lens to be focused on a screen. The mirror is located at a lower side of a foldable projection head assembly in the overhead projectors of the under mirror type. This enhances the easiness to use the overhead projectors. However, angles of both of the mirror and the projection lens need to be adjusted for the purpose of adjustment of the projection angle. Furthermore, these angles need to be adjusted so that the angle of the projection lens doubles the angle of the mirror. This construction requires an interlocking mechanism which interlocks the projection lens and the mirror for adjustment of the angles of them.
Japanese Utility Model Registration No. 3043179 discloses an overhead projector of the under mirror type provided with such an interlocking mechanism as described above. In the disclosed overhead projector, an angle adjusting knobbed bolt is turned so that the angle of the mirror is adjusted. The angle of the projection lens is adjusted by the interlocking mechanism simultaneously with that of the mirror so as to double the angle of the mirror.
In the disclosed overhead projector, however, the knobbed bolt is mounted on the projection head assembly so as to be turnable and movable back and forth. The bolt is moved to protrude from the projection head assembly when the latter is folded in a case where the overhead projector is not in use. As a result, the projection head assembly cannot compactly be folded. Furthermore, the bolt rattles when the overhead projector is carried with the head assembly folded.